How fat arms can be the sign of so many health conditions including dementia. Doctors reveal the exact circumference that puts you in danger

We've long been told to watch our waistlines. Along with a high body mass index (BMI), too much tummy fat is a red flag for health problems such as heart disease and type 2 diabetes. A BMI for adults over 25 is considered overweight, while 30 or above is obese.

But should we also be looking elsewhere for warning signs? Our arms?

If arm circumference (taken halfway between the shoulder and the elbow) is more than 32cm (12.5 in) someone is likely to be obese – and recent studies suggest carrying more fat around the arms may mean a greater risk of Alzheimer's, heart disease, fatty liver disease and even osteoporosis.

In some cases, risks were raised even when BMI was healthy.

Like the fat stored around the legs, arm fat was previously thought to be largely benign, or at least less harmful than fat stored around the abdomen – which is a sign that someone also has more visceral fat (a more harmful type that's stored in and around our organs).

Recent studies suggest carrying more fat around the arms may mean a greater risk of Alzheimer's, heart disease, fatty liver disease and even osteoporosis

However, research published a few weeks ago by scientists at Sichuan University in Chengdu, China, suggests otherwise. The researchers analysed data from 412,000 people in the UK over a nine-year period to identify any links with Alzheimer's and Parkinson's disease.

Volunteers were checked for waist and hip size, arm circumference, fat mass (a measure of their total body fat) and muscle mass (the same, but for muscle).

Those with excess tummy fat were 13 per cent more likely to develop the diseases than those with low levels.

Previous research has found tummy fat may be linked to Parkinson's (where a gradual loss of brain cells leads to tremors, stiffness, slow movement and freezing) by disrupting insulin levels and triggering a complex reaction that leads to a drop in dopamine – the brain chemical that helps regulate movement.

But the new study also found those with more arm fat were 18 per cent more at risk of Alzheimer's or Parkinson's, and also more likely to develop heart disease.

In contrast, those with muscly arms, rather than flabby, were 26 per cent less likely to develop either condition.

As a result of their findings, the researchers suggested a programme to reduce fat and build muscle in the arms may be more effective at preventing these problems than general weight loss.

The new study also found those with more arm fat were 18 per cent more at risk of Alzheimer's or Parkinson's, and also more likely to develop heart disease

These are not the only serious diseases that may be linked to arm fat. Research presented at the European Congress of Endocrinology in May showed measuring arm fat can be used to identify those at risk of a spinal fracture due to undiagnosed osteoporosis.

Researchers at the National and Kapodistrian University of Athens studied people in their 50s and 60s with no history of the disease. They found those with high levels of visceral fat had poorer quality bone in the spine, putting them at risk of a life-changing fracture.

But the same was also true for those who had fat arms, even if they were not obese according to their BMI.

In the study, the researchers used calipers, a tool that 'pinches' the skin, to measure fat lying beneath it.

It is thought arm fat could be a marker for hidden visceral fat, which is known to release inflammatory chemicals into the blood that can accelerate bone loss.

The findings could be significant because many people are only diagnosed with osteoporosis after suffering a fracture.

Eva Kassi, a professor of endocrinology who led the study, said: 'Surprisingly, we found the fat mass of arms is negatively associated with bone quality. It could mean that arm fat becomes a useful index of bone quality of the spine.'

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Meanwhile, a study by Wenzhou Medical University in China, published in May in Frontiers In Public Health, found that children aged 12 to 18 with bigger arm circumferences were 25 per cent more likely to have dangerous levels of fat around the liver, known as non-alcoholic fatty liver disease (NAFLD).

Linked to obesity and poor diet, NAFLD can lead to scarring (cirrhosis), liver failure or cancer.

So should we all be measuring our arms rather than our waists?

Not just yet, says Wasim Hanif, a professor of diabetology and endocrinology at University Hospital Birmingham, because fat in the upper arm is much less 'metabolically active' – the type linked with chronic diseases.

'The worst kind of fat is visceral fat in the belly because of its association with type 2 diabetes and heart disease,' he says. 'If I had to choose between having a big tummy or fat arms, I would always go for the arms.'

However, some researchers remain convinced arm fat is important. Professor Kassi's team are planning to expand their bone health studies to see if they can spot younger men and women – in their 30s – who might be at risk of osteoporosis, based on fat levels in their arms.

Those who are high risk will be put on exercise programmes to burn arm fat in the hope this will reverse damage to the bone.